Electrolytic apparatus for producing magnesium



R. D. BLUE ET AL April 26, 1949.

FLECTROLYTICv APPARATUS FOR PRODUCING MAGNESIUM y 2 Sheets-Sheet 1 Filed Dec. 21, 1944 \l/l/lll4lll/lll/llllllllllr 2,468,022 'NESIUM April 26, 1949. R, D. BLUE ET AL ELECTROLYTIC APPARATUS FOR PRoDUcING MAG Filed Dec. 2l, 1944 2 Sheets-Sheet 2 IIA/ff Mars Patented Apr. 26, 1949 ELECTROLYTIC APPARATUS FOR PRODUC- ING MAGNESIUM Robert D. Blue, Ralph M. Hunter, .and Marshall P. Neipert, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., arcorporation of `Delaware Application December 21, 1944, Serial No. 569,152 1 Claim. (Cl. 2011-244) The present improvements relate to electrolytic cells for the production :of magnesium by the .electrolysis of a molten body `or bath composed of magnesium chloride and chlorides of metals cal connection; (2:) to :insure a tighter and more permanent closure of the cell to prevent leakage of gases.; (3) to simplify the internal structure of the cell and reduce the .hand labor required for which are `electropositive to magnesium- EX- 5; its operation; (4) to enable partitions within the amples of :such other chlorides are those of cell, Vsuch as are now required for .separating 'the sodium, potassium and calcium, .as well as mixanode and cathode products, to be dispensed with; tures thereof. (5) to provide a close spacing of the anode and The usual fused salt baths employed for the cathode without interferingwith the separation eleCtrOlysis are heavier than magnesium, hence li() of productsand their removal from the cell; i6) to the metal floats on the surface of .the bath :as it secure great-,er Ia@essibility of those parts .of 'the accumulates durngthe eleCllIOlySS- This CiI'Cllmcell in connection with which manual operations stance has long Caused a Problem in SetSfaGlOllY maybe required during use; (7) to make feasible Separating the magnesium om the Chlorine the construction and operation of units of greater Simultaneous-1y formed by the decomposition 0f 15 capacity than has heretofore been considered magnesium Chlofde and ef/'Olved at the Surface practical in the art; (8) to reduce the power conof the bath. Various expedients hav-e been shown Sumpton por unit of product. These and other m the art for separating the products 0f the objects and advantages will be fully described in eleeerflysls Whlch generally depend upon par' the following yspecification and annexed drawing. titioning the sp-ace above the surface of the bath Azo In Said drawing: in the electrolytic cell to form separate chambers Fig .1 is an isometric .View of an electrolytc for eeueetmg the megeesmm and chlorine' The cell according to theinvention, with some cover structure of such cells is further complicated by parts removed reason ef the general .practice of euspendmg tee Fig. 2 is a horizontal section of the apparatus anodes from above `to immerse their lower end 1n 25 the b'ath. Such construction has given rise to Shgfvn Fielo wdhal ecton on the une A A a practical problem'of positioning the anodes Hite' 2 se' g l s closely enough to the cathode to reduce the ino .lg' v terna] electrical resistance of the cell to a reasonme' e 1S 'e' transverse Seetlen en the Ime B B able value and at the same time spacing the of F 1g' 2' anodes and cathodes far enough apart to permit In 'the deseen of ou? 1mpmved eleetrelyele een of a satisfactory separation of chlorinefrom ac- We have utlhzed the bl'poleir prmelple Whleh lees cumulated magnesium at the surface of the bath. been adapted to the requlrements for Werkleg Another problem has been to soa] the joints with a fused salt electrolyte of the character in where the anodes enter the cell through a cover hand- Bl'polaretectmlytlc Geus ae known and at the top, so as to prevent leakage of gas into extensively used in thel electrolysis of aqueous or out of the cell. Leakage of chlorine out of the 501111110115 Such as m the Pmqucton f f Caustc cell causes a nuisance in the cell room, while if SQda andchlofme by electfolsls 0f Sodmm 0h10 ,the ce1; is put under suction to prevent escape ride solutions, but have not hitherto been successof chlorine, air is drawn in and dilutes the 40 fully applied to the electrolysis of fused salts, chlorine. In Cells omar-gev capacity this problem particularly of fused salt baths for production of is aggravated by the large number of anodes magneslumnecessarily employed to carry the current. Each ,A b-Pelar arrangement 0f eleetrdes permits' of the anodes must be connected to the source 0f `C1`0Se Spacing of the anode and cathode sur-4 of current, vand the multiplicity of current-carry- 4.5 faces, Provided that Suitable means are employed ing leads causes stillfurther complication in the fOl limiting aS fer eS Possible the Contact 0f the cell superstructure, as well as in the operation amide and Cethde DIOdllC'GS, S0 aS t0 DI'eVen't an of the cell. excessive amount of recombination thereof. We The invention is intended to accomplish various have found that this result can be achieved to objects in dealing with the aforesaid problems, an unexpected degree by maintaining a horizontal so as to overcome or mitigate the disadvantages or limitations of thefcell structures known to the art. Among these objects are the following: (1) to obviate the necessity for a multiplicity of dependingfanodes, 'eacliwith its separate electriow of the electrolyte past the active electrode surfaces, combined with a regulated vertical circulation of the bath between the electrodes, which continuously removes the magnesium from the bathout of the zone of contact with chlorine.

The importance of these features becomes evident from a consideration of conditions existing in the space between the electrodes while the electrolysis is taking place. The chlorine evolved at the anode naturally rises in a continuous stream of bubbles, creating an upward flow of the molten bath by a gas-lift eiect. Magnesium is deposited on the cathode where it forms in small globules which grow in size until torn loose by the forces acting on them. The upow of the bath carries these detached globules to the top. This upward flow, of course, is balanced by an equal return How. If the opposing ows occur at random within the same narrow space between the electrodes, a violent churning of the bath is caused, which hinders the rise of metal globules and intermingles the cathode and anode products to an extent that results in considerable loss of metal by recombination. On the other hand,

when the return ow is diverted away from the path of the upflow so as to complete a regulated internal circuit in a vertical plane within the bath, the upward movement of the metal globules within the bath is not impeded and loss by recombination is greatly reduced. We produce such a vertical circulation by providing a neutral zone laterally adjoining the active zone of electrolysis, preferably in the direction of horizontal flow of the bath, as hereinafter more fully described.

The horizontal flow of the molten salt bath is produced by maintaining a circulation of the molten salt through the cell to sweep away the globules of metal floating on the surface of the bath. The stream of molten salt owing from the cell and carrying the metal globules is conducted to an adjacent reservoir, in which the globules coalesce into a pool of molten metal floating on the surface of the molten salt. The molten metal is removed from the pool by skimming or other means, and the molten salt, from which the metal has separated, is recirc-ulated to the cell. The inventory of molten salt is replenished as required by additions of magnesium chloride to maintain a suitable concentration of the latter in the bath.

For a more particular understanding of the bi-polar cell structure of the invention and the mode of operating the same, reference is made to the drawing, illustrating a preferred embodiment of the invention. Two rectangular chambers I and 2 (Fig. 3), separated by a wall, are interconnected at the top on both sides by conduits or troughs 3 and 4. In chamber I the electrolysis is carried out and in chamber 2 the separation of metal from the bath is ei'lected, while the circulation oi the bath between the chambers takes place through the connecting troughs 3 and d. Chambers I and 2 may be built into a `common setting 5 of brick, as shown. The inner walls and bottom of chamber are lined with a dense refractory brick resistant to the molten salt bath, while chamber 2 may be lined with steel plates welded at the joints. A suitable number of outer courses of brick is provided' for strength and heat insulation.

At one end of chamber I is terminal anode 6, which is a built-up slab of graphitized carbon set in the end wall and extending upwardly above the edge to provide a clear surface for attachment of the electrical lead-in cable l. The terminal cathode 8 of iron or steel plate is set in the opposite end wall of the chamber, and is of the same width as anode 6, but its height is such that its upper edge will be below the bath level of the cell. A connecting lead extends above 4 the setting to a terminal for attachment of cable 9.

Bi-polar electrodes I are set in alignment transversely of the chamber at spaced intervals between the terminal electrodes 6 and 8. Electrodes I0 consist of slabs of graphitized carbon having secured to the face thereof that functions as cathode an iron plate or, alternatively, an electrodeposited iron coating. An iron surface is better adapted than graphite for the electrodeposition of magnesium for the reason, well known in the art, that molten magnesium wets and adheres to an iron surface, so that during the electrolysis the magnesium forms in droplets on the face of the electrode, which grow to appreciable size before they become detached. The graphite slabs of electrodes I0 may protrude somewhat above the bath level, as shown, but the iron facings are below the surface of the bath so as not to be exposed to chlorine in the space above.

The electrodes do not occupy the full Width of chamber I. They abut on one side-wall but stop short of the opposite side-wall, and are joined to the opposite wall by panels II of refractory and electrically non-conducting material. Electrodes Ill and contiguous panels II define a plurality of parallel channels I2 running crosswise of chamber I. The portion of these channels lying between the opposing electrode surfaces is the active zone, While that between the nonconducting panels constitutes an inactive or neutral zone, which is provided for vertical circulation of the bath within each channel, for the purpose already explained.

A partition I3 in chamber 2 divides the latter into two compartments I4 and I 5 sealed olf at the liquid level, leaving the lower portion of compartments I4 and I5 in open communication with each other. In compartment I4 is pump I6 adapted to lift the liquid in chamber 2 into trough or conduit 3 at a level above that of the bath. Trough 3 `communicates through ports Il with channels I2 in chamber I, which in turn communicate through ports I8 with trough or conduit 4 leading to compartment I5, and being of sui'licient depth to extend below the bath level in the chambers.

The electrolysis chamber I is provided with a tightly sealed arched cover I9 which defines and encloses a gas space 20. In the cover is outlet pipe 2| for removal of chlorine gas. Removable covers 22 close over the top of troughs 3 and 4, and a similar cover (not shown) closes chamber 2.

For operating the electrolytic apparatus the empty chambers, with structural parts in place excepting covers which may be removed for access, are iirst heated by gas torches or other means until the walls are raised to a suflciently temperature for admission of the charge of molten salt. The covers are then put in place and both chambers are lled to the desired level by pumping in a molten salt mixture of the composition required for the electrolyte. A typical Y composition contains approximately the following percentages by weight of the respective salts:

MgCl2 22.0 CaCla 23.0 CaFz 1.5 NaCl 53.5

This salt mixture has an initial freezing point of about 610 C. Simultaneously with the introduction of the molten salt the electric current is switched on, permitting the electrolysis to begin immediately. The temperature of the bathis raised above the melting point of magnesium and thereafter maintained by the heat generated by passage of current through the bath. `Circulation of the bath between the chambers is set up by the molten salt from chamber 2 into trough 3, whence it flows into chamber I through ports il and overflows from the latter chamber through ports I8 into trough 4 through which it is returned to chamber 2. By continuously circulating the bath in this way and feeding magnesium chloride into compartment I4 to make up for that which is decomposed, the bath is maintained at approximately a constant composition and the level of the bath in both chambers also held approximately constant.

nAnhydrous, or a highly dehydrated, magesium chloride may be used for feeding the cell to replace material decomposed by the electrolysis.

A usual form of cell feed employed in other types of magnesium cells and suitable for the present purpose consists in a granular partially dehydrated magnesium chloride prepared by an ir-drying process, which contains the equivalent of from one to two mols of water of crystallization. Anhydrous magnesium chloride, either in granular or molten form, is a somewhat more advantageous, but also a more costly, type of cell feed. If desired, the operation of the present cell can be combined with the dehydration of the magnesium chloride by continuously withdrawing a portion of the cell bath 4from chamber 2 to a dehydrating apparatus in which partially dehydrated magnesium chloride isv added tothe nolten salt, thereby being substantially dehydrated and fortifying the MgCl2 content of the molten mixture. Such fortified molten salt'is returned to the cell at a rate to maintain the liquid level therein. The latter procedure has an advantage in that it permits the final dehydration of the magnesium chloride feed to be carried out in conjunction with the cell operation but in a separate apparatus where a lower salt temperature can be maintained to reduce the amount of decomposition of the salt during the dehydration.

In the electrolysis chamber I the molten electrolyte flows in a continuous stream in the channels l2 between the electrodes. Each channel functions as an individual cell, one wall of which is composed of graphite and constitutes the anode, while the other wall with its iron surface is the cathode. As already shown, the electrically 'active walls of each channel do not extend for the entire width of the chamber but are filled out by a section or panel II of non-conducting material, which is advantageously, but not necessarily, at the end adjacent to the overflow outlet. Such non-conducting portion of the walls may constitute about one-fourth to one-third of the total width of the chamber. In the arrangement shown by the drawing the electrolysis takes place in each channel in the forward portion relative to the direction of ow of the bath. At the anode surface chlorine is evolved in a continuous stream of bubbles rapidly rising to the surface of the bath, where the gas escapes into the space 20 above and is removed through the gas exit pipe 2|. This rising stream of gas bubbles induces an upward current of the bath within the channel in a direction normal to the overall direction of circulation of the cell bath, which current merges into the horizontal ow of the circulating bath and completes an internal circuit by a counterbalancing downward flow in the neutral zone between ther electrically inactive walls in the rearward portion of the channel, as shown by the arrows in Fig. 4. The magnesium globules, detached from the cathode, are swept upward by the rising current to the top, where they float on the bath and are carried away by the circulating stream through the outlet ports I8.

\ The 'circulating' stream of electrolyte leaving the electrolysis chamber and carrying along the droplets or globules of magnesium flows through trough li to compartment I5 of chamber 2, where the magnesium globules collect and coalesce to form a pool of molten metal floating on the surface of the molten salt. Such accumulation of metal is removed from time to time by any convenient means, as by dipping, siphoning or pumping. The partition. I3 keeps the metal in compartment I5 separate from compartment I4 where fresh salt is added to replenish the partly spentbath. Another function of chamber 2 is to' serve as `a settling basin for non-metallic solid substances,` such as magnesium oxide, which may be' suspended in the bath. Such oxide impurities are mostly derived from the dehydrated magnesium chloride fed to the cell, but may in part be formed by decomposition of magnesium chloride within the cell. It is an advantage of the mode of operation of the present apparatus that the continuous iiow of electrolyte past the cathode scours the surface sufficiently to prevent for the most part the adhesion of such solid nonvnetallic impurities, which foul the cathode andl require scraping off from time to time. The solid impurities settle to the bottom of chamber 2 as a sludge which may be removed from time to time by dipping or otherwise.

l The design of the present apparatus permits the bi-polar electrodes to be spaced closely for operation at a low voltage drop between successive pairs of electrodes, which is on the order of about 5 volts. The bi-polar arrangement of electrodes has the great advantage of eliminating lead-in connections for each pair of electrodes and substituting a single terminal connection at each end of a series of bi-polar units. This results in a saving of resistance losses, which may amount to as much as 0.9 to 1.3 volts for each loi-polar unit when operated at a current sufficient to maintain the working temperature of the cell. In a large commercial installation the resultant power saving represents a significant operating economy. A suitable current density is about 2.8 to 3 amperes per square inch of electrode area. In operation a current efficiency of about '75 per cent or more is obtainable, with an energy consumption of about 7 kilowatt hours per pound of magnesium produced, calculated upon the voltage measured between the terminal electrodes.

By submerging the electrodes mostly below the surface of the bath, where they are protected from contact with chlorine or small amounts of air that may be drawn in through the few crevices of the top structure, their life is greatly increased as compared with past practice. The electrodes, of course, are xed in place and cannot be replaced while the cell is in service. However, from our experience we are able to predict an electrode life of many months of continuous operation bef-ore there will be necessity to shut down and dismantle a cell unit for replacement of electrodes. This represents a substantial saving as compared with the operation of the cell types heretofore used commercially.

A particular advantage of the type of cell herein described is that it. enables a single; operating; unit to be.A designed `for larger capacity of outputy than. has heretofore been practica-l,` simply by instal-ling a suflicent number of bi--polar electrodes. Two or more electrolysis chambers, with their series of bi-polar electrodes, may be arranged in combination with a single chamber for collecting the metal and feeding fresh salt, thereby making possible further economies in cost of operation.

In addition to the foregoing advantages, all of the working parts of the cell` are easily acces.- sible. The maintenance of the cell and the removal of metal product are not attended by anyl substantial exposure of the operator to extremek versely of the chamber and parallel to each other in spaced relation, said bi-polar electrodes ex.- tending from one sidefwall of the chamber to` a point short of the opposite side-wall thereof and being connected to said opposite side-wall by meansv of a panel composed of non-conducting material, forming channels between adjacent electrodes and (zo-acting panels for flow of the molten bath, a duct leading from chamber 2 to chamber l and communicating with the latter through ports opening into the channels between the electrodes, pump means for forwarding a.

stream of the: molten. bath; through thel duct` from chamber .2 tochamber I, a return duct lead--v ing from chamber l to chamber 2 disposed at the opposite side of said, chambers from said first duct, chamber lv communicating with said return duc-t through ports; in the channels between the electrodes, said chambers being connected through said ducts in a circuit through which the mol-ten bath is circulated, means in said chamber 2 for accumulating a pool of molten magnesiumv floating on the molten bath therein, a cover over' said chamber l and a lgas outlet in said cover.

ROBERT D. BLUE'.

RALPH M. HUNTER.

MARSHALL P. NEIPERT.

REFERENCES CITED l The following references are of record in the iil'e of this: patent:

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